This invention concerns a method for self-programming a computerized control system for a power nutrunner.
The object of the invention is to provide a self-programming method for a power nutrunner control system, whereby the efficiency of the nutrunner operation is increased by decreasing the cycle time without increasing the risk for torque overshoot at tightening of so called stiff screw joints, i.e. screw joints having a steep torque growth per angle unit of rotation.
The invention relates primarily to the two-step type of tightening process which comprises a high speed first step and a low speed second step. The two steps are divided either by just a speed change or by an intermediate standstill. The first step is always the most time consuming part of the process, because it contains the relatively long running down or nut setting phase, Therefore, to bring down the time spent on the first tightening step, the rotation speed of the fist step has always been kept at a high level in prior art methods.
To safely avoid overtightening of stiff screw joints, however, the speed change point, i.e. where the first high speed step is succeeded by the second low speed step, has been set at a low torque level. The reason is that the kinetic energy or the rotating parts of the nutrunner should be prevented from having any influence on the end result, even in cases of a very steep torque growth characteristic of the screw joint.
This means on the other hand that when tightening screw joints having a very slow torque growth characteristic, this low level down shift point results in an unnecessary long and time consuming low speed second step.
The above described type of two-step tightening is commonly used, and by shifting down to the low speed second step at an early low torque point the method is safely applicable on all types of screw joints In order to speed up the process, the shift down point as well as the individual speed levels may be adjusted in relation to the characteristics of one specific type of screw joint. if, however, the system with this specific setting is used on another type of screw joint having a steeper torque growth characteristic, there is a great risk for getting an undesirable torque overshoot.
In U.S. Pat. No. 5,245,747, there is described a two-step screw joint tightening method by which the risk for overtightening is avoided by stopping the first high speed step at a very low torque level and carrying out the second step at a successively increasing speed. Thereby, the rotation speed in the second step is still low when reaching the intended final pretension level at a stiff screw joint, and this low kinetic energy in the nutruner parts does not cause any dynamic torque overshoot. A drawback with this prior art method, however, is an undesirably slow and time consuming second step when tightening weak screw joints.
In U.S. Pat. No. 5,117,919, there is described a self-adjustable nutrunner control system by which the shut-off point for the nutrunner is automatically adjusted in view of the result of preceding tightening processes. This method, however, does not comprise any detection and calculation of the actual screw joint characteristics during tightening, and the described process is performed in a single step only. Neither is there anything disclosed in this reference about how to decrease the cycle time by adjusting successively one or more nutruner operation parameters in response to empirically determined and calculated screw joint characteristics during a number of initial tightening processes.
The main object of the invention is to provide a method for self-programming a nutrunner control system during one or more initial complete tightening processes, for obtaining automatically and without any programming expertise an optimum setting of the nutrunner operating parameters for the most time efficient tightening process, irrespective of the torque growth characteristics of the actual screw joint.
A further object of the invention is to provide a method for self-programming a nutrunner control system by detecting and calculating during one or more initial complete tightening process one or more screw joint characteristics, and by adjusting successively during a succeeding number of tightening processes one or more operating parameters of the nutrunner until the tightening process is performed at a satisfactory time efficiency.
A still further object of the invention is to provide a self-programming method for a nutrunner control system intended for performing a two-step tightening process, wherein during one or more initial tightening processes one or more screw Joint characteristics are detected and calculated, and during a succeeding number of tightening processes one or more nutrunner operating parameters are successively adjusted in order to adapt the speed shift point between the high speed of the first step and the low speed of the second step, thereby extending the first step to a certain point which differs from the point corresponding to the predetermined final pretension level of the screw joint by a certain amount.
Further objects, characteristics and advantages of the method according to the invention will appear from the following specification and claims.